Alternatively, you could skip in this instance transforming the array to be all lowercase by calling toLowerCase() on each elem before you check indexOf().

some() and map() aren't supported in older IEs, but are trivial to polyfill. An example of a polyfill for each is available at the linked documentation.

As Fabrício Matté also pointed out, you can use the jQuery equivalents here, $.map() for Array.prototype.map() and $.grep() with length property for Array.prototype.some(). Then you will get the browser compatibility for free.